While this year's flu season hasn't been as brutal as past years, health experts say we still aren't in the clear — a more severe strain now accounts for nearly half of all new cases, and flu season could linger until May.

A different strain of the flu virus, a "variant" virus, now accounts for 46.9 percent of cases, the Centers for Disease Control and Prevention says. That could mean an increase in sickness: Even though more people were vaccinated this year compared to last, the current immunization protects better against the main strain, H1N1, and not as well against H3N2 viruses, according to data released Friday.

Here's what you need to know:

Strains of flu

There are four types of influenza viruses, and then subtypes of those. This flu season, health experts are mostly seeing H1N1 and H3N2 viruses. The H3 virus is considered more severe, according to the CDC.

"It looks like we are moving from an H1 wave to an H3 wave," said Lynnette Brammer, lead of CDC's domestic influenza surveillance team, HealthDayNews reports. "There's still a lot of flu to come."

Vaccines are still the best way to prevent both strains of flu. This year's vaccine is 62 percent effective against H1N1 and 44 percent effective against H3N2, Brammer, told HealthDayNews.

How long is flu season?

While the timing of flu season varies each year, the CDC estimates viruses gear up in October and November and continue as late as May. Peak flu season is typically December through February.

Symptoms

Unlike the common cold, flu symptoms hit suddenly — within a matter of hours. Symptoms include fever, feverish chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches and tiredness. Children with the flu could also vomit and have diarrhea, according to the CDC.

But, here's the kicker: It's impossible to know whether you have the flu based on symptoms alone. The only way to tell for sure is to take a laboratory test, but that's not always needed for treatment.

Flu deaths

Last week, an elementary school girl in Ohio died the same day she was diagnosed with flu and strep throat. While flu complications are rare, they do happen. This year, up to 22,300 adults have died of the flu and hundreds of thousands have been hospitalized. More than 40 children have died.

Signs of a deadly illness

The flu can cause complications that later turn fatal. Examples include pneumonia; inflammation of the heart, brain or muscle tissues; and multiple organ failure. Flu can also make chronic medical problems worse, including asthma and chronic heart disease.

Children younger than 5, senior citizens, pregnant women, American Indians and Alaska Natives are the most at risk for serious flu-related complications, the CDC says.

If you think you have the flu and are at high risk of developing complications, the CDC recommends visiting a doctor.

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